The present invention relates to multi-carrier transmission systems in which time synchronous changes in dynamic parameters are permitted, and in which a receiver clock is recovered from a pilot carrier transmitted with a fixed phase, a transceiver and a receiver for use with such multi-carrier transmission systems, and a method, for use in such multi-carrier transmission systems, of adjusting a transmitter and receiver to permit time synchronous changes in dynamic parameters.
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
In a two-way multi-carrier transmission system, such as a DMT system, in which there can be dynamic changes in the transmission parameters, some means must be provided for maintaining synchronisation between transmitter and receiver when the transmission parameters change. The first stage of such a process requires that changes of parameter be notified by one transceiver to the other, involved in an active communication process, over a slow transmission channel, i.e. the control channel. Subsequently the synchronisation of the transceivers is adjusted simultaneously, i.e. from a predetermined DMT symbol. Such adjustments in time synchronisation must be achieved with a minimum of overhead.
The present invention provides a method of maintaining time synchronisation in a multi-carrier system, without overhead, which can be implemented for a totally transparent data flow. The invention has particular application to multi-carrier DMT systems, such as MUSIC, which allow dynamic allocation of subcarrier waves, bitloading factors and other parameters which require time synchronisation.
According to a first aspect of the present invention, there is provided a receiver, for use with a multi-carrier transmission system using orthogonal carriers, in which a receiver sampling clock is synchronized with a transmitter sampling clock by extracting timing data from a pilot carrier on which a known sequence of symbols is transmitted, characterised in that said receiver includes control channel means for receiving a control channel over which data relating to changes in dynamic transmission parameters is transmitted and correlation means for extracting timing data from said pilot carrier and uniquely identifying a symbol at which a change in said dynamic transmission parameters will be effected.
Said multi-carrier transmission system may be a DMT transmission system.
Said multi-carrier transmission system may be a DMT based VDSL system.
Said known sequence of symbols may have good autocorrelation properties.
Said known sequence of symbols may be a short pseudo random sequence.
Said known sequence of symbols may have a final symbol and said final symbol may be transmitted in a DMT symbol immediately preceding an agreed symbol at which changes in transmission parameters will be effected, and said receiver may be adapted to identify said final symbol and adjust said sample clock so that frame and sample clock synchronisation are maintained.
Said transmission parameters may include bit loading, energy loading and control channel frequency.
Said receiver may be adapted to adjust said sample clock without affecting regulation of the phase thereof.
Said receiver may be adapted to receive data relating to numbering intervals transmitted over said control channel.
According to a second aspect of the present invention, there is provided a transceiver, including a transmitter and a receiver, characterised in that said receiver is a receiver as set forth in any of the preceding paragraphs.
According to a third aspect of the present invention, there is provided a multi-carrier transmission system using orthogonal carriers, in which a receiver sampling clock is phase locked to a pilot carrier, characterised in that said multi-carrier transmission system includes two transceivers as set forth in the preceding paragraph.
According to a fourth aspect of the present invention, there is provided, in a multi-carrier transmission system using orthogonal carriers, in which a receiver sampling clock is synchronized with a transmitter sampling clock by extracting timing data from a pilot carrier on which a known sequence of symbols is transmitted, a method of compensating for changes in dynamic transmission parameters, characterised by using a selected carrier wave which continuously transmits a known sequence of symbols between the transmitter and the receiver and using said known sequence of symbols to regulate the receiver""s sampling clock phase, said known sequence having a predetermined pattern of symbols with autocorrelation properties which reproduce, unambiguously, timing information which can be extracted by correlation, the last symbol of said predetermined pattern being transmitted in a transmission system symbol before a symbol where agreed changes of dynamic transmission parameters are to be implemented.
According to fifth aspect of the present invention, there is provided a multi-carrier transmission system using orthogonal carriers and including at least two transceivers, in which a receiver sampling clock is synchronized with a transmitter sampling clock by extracting timing data from a pilot carrier on which a known sequence of symbols is transmitted, characterised in that there is provided a control channel over which data relating to changes in dynamic transmission parameters can be transmitted, and in that a receiver is adapted to extract timing data from said pilot carrier by autocorrelation, said timing data uniquely identifying a symbol at which a change in said dynamic transmission parameters will be effected.
Said known sequence of symbols has a final symbol and said final symbol may be transmitted in a DMT symbol immediately preceding an agreed symbol at which changes in transmission parameters will be effected, and in that a receiver may be adapted to identify said final symbol and adjust its sample clock so that frame and sample clock synchronisation are maintained.
According to a sixth aspect of the present invention, there is provided, in multi-carrier transmission system using orthogonal carriers and including at least two transceivers, a method of synchronising a receiver sampling clock with a transmitter sampling clock by extracting timing data from a pilot carrier on which a known sequence of symbols is transmitted, characterised by providing a control channel over which data relating to changes in dynamic transmission parameters are transmitted, and by a receiver extracting timing data, from said pilot carrier by autocorrelation, which uniquely identifies a symbol at which a change in said dynamic transmission parameters will be effected.